1. Field of the Invention
This invention pertains to a dual stage dynamic rock anchor and a method of obtaining anchorage of the fixture in a rock formation in which the fixture combines a two stage complementary anchoring systems.
2. Description of the Prior Art
In a general understanding of rock mechanics a typical approach is to stabilize the roof as well as the walls of a passage formed in a geologic rock formation by forming a number of bore holes and insert anchor fixtures to stabilize the rock formation and reinforce the unsupported rock formations that have been exposed by a mining operation. The fixtures usually include a bolt or rod that is anchored in the bore hole so as to firmly hold it in place with a bearing plate and/or mats or mesh held against the wall surface by a head on the bolt or rod.
The fixtures used in stabilizing the rock formation may be of the mechanical character in which an expansion anchor device is driven into a bore hole to receive and secure a rod in position so that a thrust plate can be positioned in abutment with the rock formation exposed in the mine passage. A variation of the mechanical anchors employs a settable bonding agent to hold a rod in position, and from that stage, the next variation is made up of a combination of mechanical and bonding agents to anchor a rod in a bore hole. A further significant advance in the art of stabilizing rock formations has employed a tube of a formable material for anchoring a rod or bolt in a bore hole and maintaining the rod or bolt under tension through a plate held in abutment with the exposed rock formation.
Examples of mechanical anchors with or without bonding agents are found in U.S. Pat. Nos. 4,305,687, 4,378,180, 4,400,113, 4,516,883, 4,516,885, 4,516,886, 4,534,679, and 4,534,680. Examples of anchors secured by bonding agents are disclosed in U.S. Pat. Nos. 3,108,443, 4,096,944, 4,303,354, 4,313,697, and 4,393,638. Examples of rock fixtures employing a dry formable body or tube of material for anchoring a rod are U.S. Pat. No. 4,501,515 and French Pat. No. 1,525,224.
There are problems related to the retaining of rock formations in place when mining exposes the structure of a hitherto undisturbed rock formation. That rock structure is susceptible of collapsing under its own weight when the normal support has been removed. Thus, when underground galleries are dug, the surrounding geologic rock structure exerts on the walls of the galleries various pressures that eventually cause localized collapsing. The collapsing forces are opposed and stabilized by rock anchors inserted in the rock formations which create a series of interconnecting arches. Pin point anchoring is already known where mechanical expansion devices secure the ends of the rods deep in the sound rock formations spaced from the galleries. The anchoring rods with the expansion devices are complex and the work of installing such devices is time consuming. Others have in the past used combination anchorage systems of mechanical and resins in an attempt to develop more usable, more reliable anchors.